1. Technical Field
This invention relates generally to formation of layers on a semiconductor device, and more particularly, to formation of a thin copper layer.
2. Background Art
A common technique for forming device layers of various materials is PVD (Physical Vapor Deposition). With reference to FIGS. 1 and 2, the PVD process is a well-known magnetron sputtering process for depositing a layer on a substrate 20 supported on a platform 21. Typically (FIG. 1), a target 22 of the desired layer material is provided inside a vacuum chamber 24, and argon gas is introduced into the chamber 24 and is ionized to a positive charge (26). The target 22 is held at negative, while the wall of the chamber 24 is grounded, so that DC power is applied to add energy to the argon gas ions 26, causing the ions 26 to be accelerated toward the target 22. During the acceleration, the ions 26 gain momentum and strike the target 22. This causes atoms or molecules 28 of the target 22 to scatter in the chamber 24, with some being deposited on the substrate 20.
Formation of a thin (for example 20 angstroms thick), continuous copper layer on an oxide layer of a wafer using a copper target 29) has proven problematical. When using conventional PVD processes to form such a thin copper layer (containing only a small amount of copper) on an oxide such as SiO.sub.2 30, high surface tension causes the copper to form in large separate grains/crystals during the initial deposition, to minimize surface energy (so-called island growth). This island growth causes the deposited copper to agglomerate into distinct, separate copper globules 32 (FIG. 2) rather than a smooth, continuous, uniform layer as is desired.
What is needed is an approach wherein a smooth, continuous, uniform, thin copper layer may be formed.